Olefin (alkene) metathesis is a very well-known synthetic technique, which allows the exchange of substituents between alkenes by transalkylidenation. In recent years, metathesis reactions have been the study of intense research. Indeed, the 2005 Nobel Prize in Chemistry was awarded jointly to the chemists Yves Chauvin, Robert H. Grubbs and Richard R. Schrock “for the development of the metathesis method in organic synthesis”. 
Such redistribution of carbon-carbon double bonds is catalysed by transition metal-containing catalysts. Although other transition metal-based catalysts can be used for metathesis, such as molybdenum-based catalysts, the most common transition metal used is ruthenium, in the form of alkylidene-containing complexes (so-called alkylidene ruthenium complexes, or catalysts), more typically still alkylidene ruthenium complexes which comprise two (generally) neutral ligands and two additional anionic ligands. For a comprehensive review of such alkylidene ruthenium metathesis catalysts, the reader is referred to Ruthenium-based Heterocyclic Carbene-Coordinated Olefin Metathesis Catalysts (GC Vougioukalakis and RH Grubbs, Chem. Rev., 2010, 110, 1746-1787). In this review, emphasis is, as it typically is in the art, focused on the use of catalysts comprising carbene-containing, in particular, N-heterocyclic carbene-containing (NHC-containing) catalysts, the improved thermal and oxidative stability of such catalysts being believed to be attributable to the decreased lability of such carbenes as compared with phosphine ligands, for example, as well as other ligands coordinating through phosphorus atoms, such as phosphites, phosphinites or phosphonites. Indeed, there has been a discernible move away from metathesis catalysts comprising only phosphines as the neutral ligands in favour of carbenes, in particular N-heterocyclic carbenes.
The earliest well-defined alkylidene- and ruthenium-containing metathesis catalysts comprised two phosphine ligands and are often referred to as “First Generation” catalysts. The archetypal First Generation Grubbs catalyst is 1. Developments in this technology led to 2, the first of the so-called “Second Generation” metathesis catalysts, in which one of the tri(cyclohexyl) phosphine ligands (P(Cy)3 ligands) of 1 has been replaced with an NHC. Sometimes, including herein, catalysts of the type epitomised by 1 and 2, i.e. alkylidene ruthenium catalysts with two discrete neutral ligands are referred to as Grubbs metathesis catalysts, or simply Grubbs catalysts. Still further evolution afforded the so-called Hoveyda-Grubbs catalyst 3 (sometimes known as the Grubbs-Hoveyda catalyst), which was reported in the year 2000. This phosphine-free catalyst comprises a coordinating isopropoxy substituent attached to the aromatic ring of the benzylidene group, which replaces one of the neutral ligands. This catalyst and variants of it have proven popular owing to their improved thermal stability and oxygen- and moisture-tolerance in comparison with 1 and 2.

Olefin metathesis reactions may be divided into a variety of subclasses. These include, but are not limited to, so-called cross metathesis, ring-closing metathesis, ring-opening metathesis polymerisation (often referred to as ROMP) and self metathesis reactions.
Cross metathesis appears to be subject to a variety of definitions in the literature, including for example a metathesis reaction between two non-cyclic olefins, and an intermolecular metathesis reaction between terminal alkenes. However, cross metathesis as defined herein is any metathesis reaction between two alkenes. Typically the two alkenes participating in a cross metathesis will be acyclic. It will be understood that, where the participating alkenes are the same, such a cross metathesis reaction is an example of self metathesis. Typically, however, cross metatheses are not self metathetic.
Ring-closing metathesis is a reaction whereby a ring is formed as a result of a metathesis reaction between two carbon-carbon double bonds. For example, an acyclic diene, typically in which the two participating C═C bonds are terminal may be ring-closed. In contrast, ring-opening metathesis polymerisation involves, as the name implies, both ring-opening of a cycloalkene and polymerisation of the resultant diene.
Each of these (and other) classes of metathesis reactions are well-known to and understood by the skilled person and, as discussed above, may be and often are catalysed by alkylidene ruthenium complexes.
G S Forman et al. (Organometallics, 2005, 24, 4528-4542) report enhancement to the performance of certain olefin metathesis reactions catalysed by Grubbs catalysts by the simple addition of phenol or a substituted phenol. In a published patent application (WO 2004/056728 A1), similar metathesis reactions are described. In neither of these publications, however, is it in any way described or contemplated that the substituted phenol may be tethered to a C═C bond participating in a metathesis reaction, in other words that a phenol-comprising molecule participates in a metathesis reaction.
J A Mmongoyo et al. (Eur. J. Lipid Sci. Technol., 114, 1183-1192 (2012)) describe a specific example of a cross metathesis reaction between ethylene and cardanol. Cardanol is a term used to refer to a mixture of compounds each of which is a phenol having a C15 hydrocarbyl straight chain at the 3-position and which vary in the degree of internal unsaturation in the chain, which has between 0 and 3 carbon-carbon double bonds. The cross metathesis reaction described is catalysed by the Hoveyda-Grubbs catalyst (3, supra) is described as providing a less than perfect yield, with the reaction giving undesired quantities of other products believed to result from a series of side or competing reactions.
The type of scissile cross metathesis reaction described by J A Mmongoyo et al. Is sometimes referred to as ethenolysis, since the metathesis reaction between ethylene and an internal double bond serves to cleave the internal C═C bond.
In a recent study (R. M. Thomas, B. K. Keitz, T. M. Champagne and R. H. Grubbs, J. Am. Chem. Soc., 2011, 133, 7490-96), Grubbs-Hoveyda-type catalysts containing unsymmetrical N-heterocyclic carbenes are described in which one N substituent is alkyl and the other aryl. These are described as highly selective catalysts for the ethenolysis of methy oleate, the best having di(2-propyl)phenyl and norbornyl substituents.
As is known (see for example the discussion by Thomas et al. (immediately supra), and the references cited therein), ethenolyses are particularly challenging metathesis reactions to conduct. This is believed to be because the catalytic cycle involves reaction between a methylidene complex and the internal alkene being subjected to ethenolysis. Many alkene metathesis catalysts are understood to be unstable as methylidene complexes and it is degradation of these intermediate species that is believed to give rise to the difficulties encountered with ethenolysis reactions.
There is a continual need for modifications and/or improvements to existing ethenolysis methodologies and the present invention addresses this need in the art.